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in a previous video we described the
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dhcp process as seen from the
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perspective of the workstation but what
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are the configuration settings that we
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need to make inside of the dhcp server
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itself well first we'll need an ip
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address range we'll need to understand
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exactly what ip addresses will be
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assigned by this particular dhcp server
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and we'll need the associated subnet
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mask for that ip address range we'll
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also need to determine how long a
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workstation can hold on to the same ip
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address will make that configuration
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setting under the lease duration and
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we'll also want to configure the dhcp
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server with dns server settings so that
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your in stations can be configured with
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an appropriate dns server ip address a
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default gateway setting and if you're
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using things like voice over ip servers
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be nice to include the options for that
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along with all of the other ip
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configurations
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when the dhcp server assigns an ip
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address to a device it's choosing an
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available address from a pool of
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addresses that you've previously
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configured inside of the dhcp server for
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example you might have one subnet pool
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that is 192.168.1.0.20
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which means anything in that subnet can
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be assigned as an ip address from the
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dhcp server these ip scopes are usually
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a very large contiguous range of ip
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addresses and your dhcp server simply
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pulls from any available address inside
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of that range but there may be times
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when you'd like to set a dhcp
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reservation or exclude certain ip
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addresses from that range and you can
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certainly create those exceptions within
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the dhcp server
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here's a dhcp server that's running on a
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windows server device this specifies the
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scope of 165.245.44.0
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and underneath that scope we have a
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series of address pools so we know what
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ip addresses will be assigned we have
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address leases so we can view what i p
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addresses have previously been assigned
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we can configure ip reservations so if a
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certain device should always receive the
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same ip address we can configure that in
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that section of the dhcp server and we
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have scope options which allow us to
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configure additional parameters for
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example we might want to add the ip
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address of a voice over ip gateway so
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that all of the devices on your network
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would know exactly what ip address to
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contact
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if you're using dhcp at home or in a
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small office your dhcp server may not be
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on a windows device it may be on an
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embedded router this is the web front
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end to an embedded router that shows
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that the dhcp server is enabled the
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start address on this network is
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10.10.10.2
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and the end address is 10.10.10.100
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so we can expect this dhcp server to
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assign addresses starting with the dot 2
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all the way up to dot 100 this dhcp
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server specifies the address lease time
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in seconds which is a bit unusual but if
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we know that 86 400 seconds is the same
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as 24 hours this configuration works
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just fine the gateway is configured as
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10.10.10.1
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and we have dns configurations that will
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also be assigned to our local devices so
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any device on your network that needs an
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ip address when it starts up will
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receive an address from this dhcp server
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with these specific configuration values
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for most devices connecting to a network
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and receiving a dhcp address they're
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receiving a dynamic assignment this
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means they could receive any ip address
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from that large pool of ip addresses
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that we've previously configured and
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after your lease period has timed out
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those addresses will be available for
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another person who connects to the
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network many dhcp servers will also have
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an automatic assignment where they will
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keep a list of everyone who's previously
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connected to the network and if you
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happen to connect to the network again
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after a short period of time away it
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will remember your previous assignment
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and if that ip address is still
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available it will assign you the same ip
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address you had originally
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you can also take this one step further
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by configuring a dhcp address
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reservation which means that a device
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connecting to the network will always
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receive that same ip address and that ip
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address will never be given to a
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different device on the network this is
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usually configured based on the mac
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address or media access control address
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which is the burned in address on a
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network interface card every device has
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a unique mac address and that allows us
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to associate a device with a particular
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ip address you might also see this
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referred to as a static dhcp assignment
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static dhcp a static assignment or an ip
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reservation
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here are some dhcp reservations in my
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dhcp server on a soho network you can
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see a mac address is listed along with
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an ip address and the host name for this
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device is prometheus i have another
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device listed by mac address you can see
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that is a different mac address in the
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original it gets a different ip address
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and the host name for that device is
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odyssey this means that when prometheus
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starts up it will always receive the ip
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address of 192.168.1.6
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and that ip address will never be
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assigned to another device on this
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network
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when a device is dynamically assigned a
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dhcp address it's a temporary assignment
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after a certain amount of time that
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device must check in again to the dhcp
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server to let it know that it is still
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working with that same ip address or the
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lease will timeout and that ip address
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will be available for others the amount
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of time in that lease is determined by
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the configuration within your dhcp
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server we saw on my dhcp server the
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lease times are 24 hours but you can
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administratively configure that to be
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any value you'd like there's also a
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reallocation process that can occur if
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you reboot a device or you leave the
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network and then return to the network
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it will renew that lease and continue
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using the same ip address and there may
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be times when you would like to
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administratively or manually release
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that ip address hand it back to the dhcp
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server and then either leave the network
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or request a new address from the dhcp
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server
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when a device receives an ip address
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from a dhcp server there's a timer that
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starts that is the length of the lease
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time that's configured for that dhcp
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server we mentioned earlier that once
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that lease timer has expired that ip
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address is returned to the dhcp server
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and it can be assigned to others but
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during that lease process there are
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other timers you should know about one
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is called the t1 timer this checks in
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with the dhcp server halfway through the
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lease time so if your lease time was
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eight days this t1 timer which is 50
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percent of the lease time by default
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will check in after four days and let
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the dhcp server know that we would like
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to keep this ip address for another
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lease duration at that point the timer
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resets to eight days and we start
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counting down again
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there may be times though that a dhcp
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server is unavailable and you're not
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able to check in with that t1 timer
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after four days if that dhcp server
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never returns to the network then the
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lease time continues to count down and
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once you get to 7 8 of that lease time
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or 87 and a half percent of that lease
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time it will try rebinding with any
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other dhcp server that you might have on
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the network so that it can retain that
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ip address this is the t2 timer and it
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gives every device on the network a
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chance to keep its ip address by
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rebinding with a redundant dhcp server
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let's look visually at how this dhcp
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process might occur let's say in this
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particular network the lease time is 8
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days in length that means your t1 timer
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if it's 50 percent of that time would be
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four days and a seven eighths timer of
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t2 would be seven days in length so
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let's look at this device which has been
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given an ip address and each one of
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these blocks is a single day somewhere
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after the fourth day or after 50 percent
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of this time has gone by and the t1
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timer has gone off it will want to renew
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this ip address by contacting the dhcp
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server and letting it know that it would
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like to renew that particular lease once
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that process is complete the timer
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restarts and we have another dhcp lease
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process that will occur
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however during the second lease process
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our original dhcp server is no longer
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available so we're not able to check in
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after that t1 timer has completed this
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means that we'll go all the way into the
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rebinding period with the t2 timer or in
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this particular case after seven days
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have elapsed once we're able to contact
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that redundant dhcp server after the t2
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timer has expired the process then
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begins again starting at day one and we
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have another eight days in our lease
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you
